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Role of R5 and X4 HIV strains in disease progression. A) Persistence R5 HIV-1 strains at all stages of HIV infection particularly at the late stages of HIV infection whereas X4 HIV-1 strains emerge at the advanced disease stage only; B) the course of HIV infection and disease. Levels of CD4 and viral load are shown to correlate with the progress of HIV infection and CD4+ T cell depletion.
